1. Field of the Invention
This invention pertains to telephones in general, and in particular, to an emergency, or backup, power supply for a telephone set that does not use batteries.
2. Description of Related Art
Conventional telephone sets, i.e., those coupling to the “tip and ring” terminals of a conventional analog “local loop” of a “Plain Old Telephone Service” (“POTS”) or a PBX, typically comprise a “transmitter” (e.g., a mouthpiece or microphone), a “receiver” (e.g., a speaker or earphone), and a manually actuated “switchhook” mechanism that switches the set between an “off-hook” condition, i.e., one coupled to the local loop for signaling and communication purposes, and an “on-hook” condition, i.e., one decoupled from the local loop. The analog current-signaling technique employed in such telephones enables them to operate without need for an external power supply, since all of the electrical power needed to operate the set, 48 VDC at up to 120 mA, or 5-6 W, is supplied on the local loop by the local telephone company (“Telco”) or PBX so long as the telephone set is in the off-hook condition.
However, when the set is in the on-hook condition, i.e., decoupled from the local loop, the maximum amount of power that the set can draw from the local loop is substantially limited, by FCC regulation, to 480 μW, i.e., 48 VDC at 10 μA. Accordingly, telephone sets that incorporate enhanced features, such as “hands-free” operation (i.e., “speakerphones”), dialed-number and program and data memory storage, Dual Tone Multi-Frequency (“DTMF”) signaling, automatic dialing and answering, and switching control of ancillary equipments, such as video security cameras or entryway door locks, typically all require some form of an external power supply. This is because the electronics necessary to implement these additional features, such as microprocessors, dynamic memories, amplifiers, DTMF transceivers, solid-state hybrids, ring detectors, and the like, all require a moderate amount of electrical power to function, and while the power supplied by the local loop may be sufficient to power these electronics when the set is off-hook, it is inadequate to power them when the set is on-hook.
Accordingly, enhanced-function telephone sets typically incorporate a power supply in the form of an AC-DC converter that connects to the local power “mains” for operation of the set during normal operating conditions, and a battery that functions as a temporary, or backup, power supply for the set in the event of a power outage. Indeed, certain special-purpose, or “mission-critical,” telephone sets, e.g., “emergency” and “access-control” telephones, which are required to at least receive or place voice calls during emergency conditions involving power outages or interruptions, are typically provided with a power supply that comprises an AC-DC converter, a rechargeable backup battery, and a battery-charging circuit that maintains a full charge on the battery during normal operating conditions.
However, backup power supplies that comprise batteries include several drawbacks that affect their long-term reliability. For example, batteries that are not rechargeable, such as alkaline or lithium batteries, must be replaced periodically if the backup power supply, and hence, the telephone set, are to function reliably in an emergency. Rechargeable batteries, such as nickel-cadmium, nickel-metal-hydride, or lithium-ion batteries, are relatively larger and more expensive than non-rechargeable batteries, incorporate materials that are hazardous to the environment, are “current-limited,” and therefore require relatively sophisticated re-charging circuitry, take relatively long periods of time to recharge, and are limited in the number of times that they can be recharged before they need to be replaced.
Accordingly, a long-felt but as yet unsatisfied need exists for a backup power supply for a telephone set that is smaller, less inexpensive, safer for the environment, longer-lived, and more reliable than power supplies that rely on batteries for the storage of temporary backup power.